There are many well developed techniques for the bulk removal of dissolved impurities from water solution. Some typical examples are evaporation, ion exchange and reverse osmosis. These techniques are generally less well suited to selectively removing certain particular impurities while leaving the remainder of the dissolved constituents in solution. However, selective removal of minor contaminant ions from solution is a very common requirement for many applications, particularly when processing radioactive effluents from nuclear power plants and other facilities. In this case the removed contaminants require careful containment or disposal as radioactive waste, and the volume of such waste must be rigorously minimized for economic and other reasons. If harmless dissolved species are removed concomitantly with the radioactive contaminants, the resulting waste volume will be uneconomically large. Effluents usually contain radioactive species in very low molar concentration (typically about 10−15 to 10−12 moles per liter) and the chemistry of the radioactive ions is usually distinctly different from other harmless dissolved species present in greater concentration. This permits the design of specific removal methods to target the radioactive ions only.
The most typical way of removing contaminants from solution is to transfer the contaminants to a different phase, normally from liquid to solid. If the radioactive ions bind onto particles added to the solution, the particles can be removed by solid liquid separation techniques. There are many examples of selective solid absorbers which remove radioactive ions, including industrial scale applications such as the use of clinoptilolite for the selective removal of cesium and strontium from effluents from the Sellafield plant in the UK.
There are, however, problems to be overcome in designing a selective radioactive ion removal process as described above. In order to have adequate capacity to hold the contaminants, the particles must either be porous (so that the absorption functionality is distributed throughout the volume of the particle) or they must be very small. Robust porous particles, (like clinoptilolite above), are difficult to create and usually, like clinoptilolite, have limited selectivity of absorption. More choice of selective absorbers is available with small particles, which may also have the potential benefit of rapid absorption kinetics (which may be very important in designing a practical process plant). However, small particles are difficult to separate from solution by solid liquid separation techniques. This final difficulty may be overcome by creating special small magnetic particles, which can be separated from solution by magnetic filtration. If the particles are made highly selective for contaminant removal, only small quantities of the particles will be required, thereby allowing various separation protocols within normal economic constraints.
The general principle of selective removal of radioactivity by magnetic particles was previously explored wherein the particles were built up by surrounding magnetic cores (of magnetite, for example) with an organic polymer onto which could be attached selective ion exchange functions, or other particles of selective ion exchangers could be embedded in the forming polymer. As a consequence of the production methodology, (that is, using solid magnetic particles as the core and emulsion polymerization methods), the particles had a diameter of about 10 to about 100 microns, and it is not possible to reduce this significantly. At the time of this earlier work, another reason for the large particle diameter was that the particles had to have a core size sufficient for the magnetic forces acting on the particle to cause efficient migration of the particles through the liquid. Since that time magnetic filtration technology has improved considerably, and much smaller particles can now be efficiently filtered from solution.
Because of the relatively large particle diameter, the ion exchange capacity was small. The ion exchange functionality was only present on the surface of these particles, not throughout their volume. To overcome this the absorption on the particles was usually made reversible, so that the contaminant could be removed from the absorber and the absorber reused. However, the requirement for reversibility of the absorption reaction placed limitations on the choice of the selective absorber used, and limited the selectivity achievable for any specific contaminant. The capacity limitation therefore restricted the usefulness of the concept to a few special applications.
In a totally different field of science developments have been taking place in the production of small magnetic particles. The need for ever more compact information storage systems has led to production of uniform magnetic particles of diameters of the order of 10 nanometers (0.01 microns). The intricacy of biological systems has been harnessed to this purpose. In the biological systems, iron is required for uses such as hemoglobin production and is typically stored in protein conglomerates called “ferritins”. These naturally synthesized protein conglomerates have the desired dimensions and have a central cavity which accumulates iron in the form of oxide/hydroxide. Furthermore the iron can be removed and replaced from the cavity either under natural conditions, or under laboratory control. The natural iron can also be replaced with intensely ferromagnetic materials, thereby improving information storage properties or, for the present purposes, aiding the subsequent magnetic filtration. The removal of these non-functionalized enhanced magnetic ferritins (magnetoferritins) from water solution has been demonstrated using modern magnetic filtration methods.